1. Field of the Invention
The present invention relates to a vacuum die casting method and system and, more particularly, to a system and method in which multiple die casting machines are connected to and function with a single vacuum apparatus resulting in a simple, efficient and low cost arrangement that produces better and stronger castings.
2. Description of the Related Art
Die casting is based upon the principle of forcing molten metal into a mold or die under considerable pressure. This technique makes possible the economical production of intricate castings at a rapid rate. Such castings, which, for example, may comprise various holes, recesses, screw threads, and shapes, are characterized by high dimensional accuracy, good surface finishes and economy of metal. Such castings require little or no final machining.
A typical cycle of operation comprises closing the die, using a plunger to force forcing the molten metal into the die cavity, withdrawing any cores, pausing for a predetermined time, opening the die, ejecting the casting, shearing off the sprue, deburring the casting and cleaning the die. The number of cycles per hour that a casting machine can attain will depend on the size and shape of the castings and on the casting metal used. Typically, casting dies are very expensive because they are precision made and they are of quality materials to survive severe working conditions characterized by high pressures and numerous variations in temperature. For the production of zinc and zinc alloy castings, the dies may be made of unalloyed steel. For aluminum, however, and for magnesium and copper and the alloys of these metals, the dies are usually made of hot worked tool steel which has greater durability.
More recently, a pressure reducing or vacuum apparatus has been developed which cooperates with a die casting machine to produce even stronger castings with finer surfaces. When using a vacuum apparatus, the air in the system is mostly evacuated, and the metal is pulled into the die as well as pushed. Typically, the vacuum apparatus includes a pump, an electric motor, a receiver and a hose.
Most recently, vacuum die casting systems have been joined to programmable logic controllers, valve assemblies, actuator assemblies and related process piping and electrical connections. Signals from the die casting machine, including the position of the plunger and a start-of-cycle signal are sent to the controller which in turn signals the vacuum apparatus to assist in the casting process. Upon receiving a start-of-cycle signal from a die casting machine, the vacuum system is energized. The valve is opened to expel excess metal from the previous cycle that may be present and to test for blockage around the valve and to test the vacuum hose. The valve is then closed and the receiver and the hose is evacuated. The pump is isolated from the system and a test for pressure decay is performed. If either of two these tests show a problem, a light on a control panel is lit and the die casting machine ceases operation. While the testing is undertaken, the casting from the previous cycle is removed from the die cavity and the die is sprayed with a release coating for the next cycle.
After the tests have been completed, the die is closed and locked and the molten metal, usually aluminum, is poured into a chamber. The injection plunger is energized and moved slowly past the pour hole. Once the pour hole has been sealed, a signal is sent to the vacuum apparatus. The vacuum valve is opened by the actuator allowing the removal of air from the die cavity and chamber. As the plunger movement continues, the low pressure is maintained. If the low pressure level cannot be held below a preset level, a signal is produced and the casting cycle is halted. Before the molten metal, which is being forced and sucked into the die cavity, reaches the vacuum valve at the far end of the flow path, the actuator allows the valve to close.
While vacuum die casting has certain advantages over ordinary die casting, not all castings need be exposed to a vacuum or low pressure operation, and maintaining a full system including a die casting machine and a vacuum apparatus is a considerable investment, especially when a manufacturer has multiple die casting machines. Trying to enhance efficiency in these circumstances proved to be an interesting task which has heretofore escaped solution.